The present invention relates to a method of assessing positional uncertainty in drilling a well.
In order to drill a well, it is necessary to define a geological target for the placement of the well. The geological target is a surface which is bounded by geological factors such as the position of geological faults and the extension of an oil-water contact. The geological target is defined by a geophysicist and is based on data about geological structures. Such data may be obtained, for example, in the form of seismic data or as data from nearby existing wells.
Some geological target boundaries are more important than others in the sense that it is more important to be inside some boundaries than others. For example, if a drill bit misses an oil zone, it will never be possible to produce oil. The geophysicist thus defines a reduced geological target whose boundaries are judged to be sufficiently remote from the boundaries of the geological target to ensure that there is a very good chance that the wellbore will not stray outside the geological target.
FIG. 1 of the accompany drawings illustrates such a conventional geological target 1 in the form of a rectangular surface having boundaries 2 to 5. Each of the boundaries 2 to 5 is associated with a risk in the form of a percentage associated with the drill bore straying outside the boundary. Thus, the risk of straying outside the boundary 2 should be no greater than 1% whereas the risks of straying outside the boundaries 3 to 5 should be no greater than 2.5%.
Within the conventional geological target 1 shown in FIG. 1, various geological structures are illustrated by way of example. A conventional reduced geological target 6 is also illustrated and this is defined by the geophysicist on the basis of experience.
Thus, the geophysicist judges how far the boundaries of the conventional reduced geological target 6 should be spaced from the boundaries of the conventional geological target 1. Because of the higher risk associated with the boundary 2, which corresponds to a geological fault, the corresponding boundary 7 of the conventional reduced geological target 6 is more remote than the boundary 8 with respect to the corresponding boundary 4.
The xe2x80x9crisk valuesxe2x80x9d shown in FIG. 1 as percentages are effectively the inverse of the acceptable probabilities of straying outside the respective boundaries. These values are generally referred to as xe2x80x9chardline valuesxe2x80x9d and risks or probabilities are conventionally only assigned to boundaries which must not be crossed.
The geological data about the nature and location of structures beneath the surface of the earth are not precise; if such data were precise, then there would be no need for the conventional reduced geological target. There is a degree of uncertainty in the actual position of geological structures compared with the positions indicated by seismic and other data. This results in the need for the reduced target, whose purpose is to set an actual target for a driller to aim for during drilling of the well. The actual uncertainty in position varies from situation to situation but it is possible to provide some measure of the inaccuracy of the geological data. The geophysicist uses judgement in deciding the size and location of the conventional reduced geological target 6 within the conventional geological target 1.
Drilling of a well is also not a precise process. The geophysicist supplies the conventional reduced geological target 6 to a drilling engineer who must then define a drillers target within the conventional reduced geological target 6. The actual position of a drill bit compared with the measured or estimated position is also subject to inaccuracies. Such inaccuracies depend, for example, on the well trajectory geometry and the accuracy of drill position measuring equipment located behind the drill bit. The position measuring equipment can provide measurements of different accuracies depending on the type of measuring equipment and, in particular, on the cost thereof. A typical drillers target is shown at 9.
The drilling engineer has to define the drillers target such that, if the position of the drill bit is measured to be inside the drillers target, there is a predetermined likelihood that the well will actually be within the conventional reduced geological target 6 and hence, allowing for the inaccuracies in the geological data, the actual positioning of the well will be acceptable. The drilling engineer must judge whether more money should be spent on the drill position measuring equipment in order to improve the chances of drilling the well in the correct place.
The present invention may be characterized as a method of assessing positional uncertainty in drilling a well. Such a method may be used, for example, at the planning stage in order to direct the drilling operation and to assess whether it is worth while to drill a particular well. The method may also be used in real time to control the drilling of a well.
According to a first aspect of the invention, there is provided a method of estimating positional uncertainty in drilling a well, comprising supplying a first set of values representing a first three-dimensional uncertainty of the actual position of a drill bit with respect to the estimated position thereof, supplying a second set of values representing a second three-dimensional uncertainty of the actual position of a geological feature with respect to the estimated position thereof, combining the first and second sets of values to form a third set of values representing a third uncertainty of the position of the drill bit with respect to the geological feature, and calculating from the third uncertainty the probability that the drill bit reaches a predetermined position relative to the geological feature.
At least one of the first, second and third sets of values may comprise parameters of an error ellipsoid with a predetermined confidence interval referred to a Cartesian coordinate system.
At least one of the first, second and third sets of values may comprise a covariance matrix referred to a Cartesian coordinate system.
The first and second sets of values may be referred to different coordinate systems and the combining step may comprise transforming the first and second sets of values to fourth and fifth sets of values, respectively, referred to a common coordinate system and summing the corresponding values of the fourth and fifth sets to form the third set of values.
The probability may be calculated as a normal distribution.
The method may comprise defining a geological target as a finite surface and selecting a desired point of intersection of the drill path with the geological target. The method may comprise calculating the probability of the drill path intersecting the geological target. The geological target may be a polygon. The geological target may be rectangular. Each side of the polygon may be ascribed a maximum acceptable probability of the drill path missing the geological target on that side.
The method may comprise calculating the probability of the drill bit being at a predetermined distance from the geological target.
The method may comprise using information from a marker point whose relative position including positional uncertainty to the geological target is at least partly known to correct at least one of the first set of values. The marker point may be the position of the drill bit during drilling when the drill bit penetrates a seismic reflector whose distance from the geological target is at least partly known. The geological target may be selected to coincide with a predetermined geological structure, the marker point may be disposed at the predetermined geological structure, and the position of the predetermined geological structure may be derived from a pilot well. The marker point may be observed during drilling using means disposed at or adjacent the drill bit. Such means may, for example, comprise seismic, acoustic or electromagnetic means. The method may comprise defining a drill target as a sub-surface within the geological target and calculating the probability that the drill path directed at a point within the drill target will intersect the geological target. The method may comprise defining a drill target as a sub-surface within the geological target and calculating the lowest probability that the drill path directed within the drill target will intersect the geological target.
The method may comprise defining a drill target as a sub-surface within the geological target and calculating the total probability that the drill path directed within the drill target will intersect the geological target.
The method may comprise deriving a drill target as a sub-surface within the geological target whose boundary is defined by a predetermined probability.
The method may comprise defining a plurality of geological targets along an intended drill path, calculating the probability of the drill path intersecting each of the geological targets, and deriving from the calculated probabilities the probability of the drill path staying within a corridor defined by the geological targets.
According to a second aspect of the invention, there is provided a method of assessing the value of a well, comprising supplying details of a hydrocarbon reservoir, selecting an optimum point of intersection of a drill path with the reservoir, calculating the probabilities of the drill path intersecting the reservoir at a plurality of points using a method according to the first aspect of the invention, and calculating the probability distribution of the value of recoverable hydrocarbons for each of the points of intersection and deriving from the calculated probabilities and the probability distribution a distribution of the value of the well.
The drill may be partially withdrawn and the direction of drilling may be changed if the probability of the drill path intersecting the geological target following correction of the first set of values is less than a predetermined value.
It is thus possible to provide a technique which allows the uncertainties in the drilling of a well to be quantified in terms of probability. For example, when planning the drilling of a well, a geological target may be determined in the usual way with the appropriate hardline values being selected for the boundaries. Uncertainties in the actual positions of geological features compared with estimated or measured positions and uncertainties in drill bit position compared with estimated or measured position are combined to allow probabilities to be given, for example as to whether a selected intersection point with a geological target will be achieved. This allows the drillers target to be defined more accurately so as to improve the probability of correctly positioning a well. Also, the degree of accuracy of measurement of the drill bit position can be selected so as to achieve an acceptable probability of correctly positioning a well.
When combined with details of a hydrocarbon reservoir, it is possible to assess the commercial viability of the well and the need for more accurate drill bit positioning equipment when drilling the well. For example, if the structure of the reservoir is known or estimated, for example from geological data, the profitability of the well can be plotted as a function of probability and vice versa. The profitability of the well can be measured as the value of the hydrocarbon reserves which can be produced for a given position of the well head at the hydrocarbon reservoir minus the costs of production. The probability of the position of the well head can be assessed. This allows more informed decisions to be taken as to whether it is commercially worth while to extract the hydrocarbon reserves and what sort of measuring equipment should be used during drilling of the well.
These techniques may be used during the planning stage before beginning to drill a well. However, the present technique may also be used in real time during drilling. For example, the material withdrawn through the drill string during drilling can indicate when the drill bit has reached the position of a known type of rock. At that point, the position of the drill bit is known to greater accuracy and this can be used to correct the set of values representing inaccuracy of the position of the drill. Such information may be used to guide the drill so as to increase the probability of intersecting the geological target at a particular position. It may be determined that the drill is straying too far away from the desired trajectory, in which case the drill may be steered so as to return towards the desired trajectory. If the drill bit has strayed too far away from the desired trajectory for correction by steering to be possible, it is possible to withdraw the drill bit partially and then to recommence drilling in a different direction so as to return towards the desired trajectory.